The present invention generally relates to drilling fluids, and more specifically, to methods for drilling in a subterranean formation and drilling fluid compositions adapted for the same.
Subterranean wells are drilled for a number of applications, including oil and gas exploration (e.g., wireline core drilling), water exploration, and mineral exploration. In a typical drilling operation, drilling fluid is used during drilling, for example, to control formation pressures, seal permeable formations (e.g., form a filter cake), remove cuttings from the well, and cool and lubricate the drill bit. Although the function of the drilling fluid is generally the same across all applications (e.g., oil and gas, water, and mining), the properties of the drilling fluid must be adapted to each particular application. For example, mining applications, particularly mining exploration applications, tend to involve smaller wellbores and, thus, smaller drill bits, than oil and gas applications. Moreover, drilling during mining exploration tends to involve highly mineralized formations and the integrity, pH, and temperature of these formations may be vastly variable. Moreover, drilling during mining exploration is characterized by and small annular spaces between the formation and the drilling apparatus. Therefore, such applications may require less viscous drilling fluids such that the pressure exerted by the drilling fluid does not damage the formation or drilling fluids that do not vary in its consistency with changing pH or temperature.
Traditional drilling fluids use clay derivatives (e.g., bentonite) that help build the filter cake and control water loss during drilling. They can also be used to suspend cuttings created during drilling and carry them out of the wellbore. However, clay derivatives alone can become too viscous to be adequately pumped and can produce a thick, unstable filter cake, which can create undesirable drag during drill bit pullback. Therefore, suspension enhancers are often added to drilling fluids with clay derivatives in order to reduce the amount of clay derivatives that must be used. In some instances, suspension enhancers additionally serve as water loss reducers. In traditional drilling fluids, suspension enhancers are not used in drilling fluids alone because they do not provide adequate fluid loss control or filter cake production in all formation and water types.
Traditional drilling fluids must be prepared on site in order to account for the variability in formation properties and which may significantly delay drilling operations. Additionally, a particular order of operations for including additives is necessary to ensure that the drilling fluid functions properly for the particular application. For example, if a drilling fluid contains a clay derivative and suspension enhancer (e.g., polymer), it is necessary to hydrate the clay derivative before adding the suspension enhancer in order to ensure that the clay derivative is capable of maximum swelling. If the suspension enhancer is added to a drilling fluid before the clay derivative, the suspension enhancer will interfere with the grains of the clay derivative and prevent it from properly swelling, thereby rendering the drilling fluid less effective. Moreover, the suspension enhancers selected for use in a drilling fluid must be compatible with the particular application (e.g., a polymer that loses its water loss control capacity at high temperatures should not be used in a formation that has high temperature intervals). Also, operator error in mixing drilling fluids tends to lead to inconsistent blends.